Spacecraft propulsion systems generally include thrusters, which may be broadly categorized as either “chemical” or “electric” based on the respective primary energy source.
Chemical thrusters, whether the propellant is solid or liquid, monopropellant or bipropellant, deliver thrust by converting chemical energy stored in the propellant to kinetic energy delivered to combustion products of the chemical propellant. Chemical thrusters, as the term is used herein, and in the claims, also include electrothermal thrusters such as arcjets, described for example in U.S. Pat. Nos. 5,485,721 and 5,819,526, that are configured to use electrical energy to increase the temperature, and, therefore, the velocity of the combustion products of chemical propellants.
In contrast, an electric thruster, as the term is used herein, and in the claims, converts electrical energy to propellant kinetic energy substantially without regard to any chemical energy the propellant may possess. For example, an electric thruster may operate by ionizing and accelerating a gaseous propellant, where the propellant is a noble gas of a heavy element, such as xenon or argon. Irrespective of the selected propellant, a negligible amount of thrust results from energy chemically stored in the propellant. The term electric thruster, as used herein and in the claims, encompasses an electrostatic thruster, an electromagnetic thruster, a Hall effect thruster, a wakefield accelerator, and a traveling wave accelerator, for example.
Chemical thrusters suitable for spacecraft propulsion systems may deliver relatively high thrust of 10-1000 newtons, for example, substantially irrespective of spacecraft power limitations, but such thrusters are generally incapable of operating at a specific impulse (Isp) higher than 500 seconds. Electric thrusters may operate at an Isp of 1000-4000 seconds, but spacecraft power constraints, at least, practically constrain thrust levels to well less than one newton.
During the course of a typical spacecraft mission there are times that a high thrust, low power thruster is desirable; at other times, however, a high Isp thruster is more advantageous. As a result, it is known, as illustrated in FIG. 1, to provide both chemical and electric thrusters on board a single spacecraft, each thruster assigned to a propulsion subsystem having its own dedicated propellants and its own dedicated propellant supply arrangements.